## Loading required package: flowWorkspaceData

An Introduction to openCyto package

1. Introduction

The openCyto package is designed to facilitate the automated gating methods in sequential way to mimic the manual gating scheme.

1.1. Manual gating

Traditionally, scientists have to draw the gates for each individual sample on each 2D projections (2 channels) within flowJo. Or draw the ’template gate’s on one sample and replicate it to other samples, then manually inspect the gate on each sample to do the correction if necessary. Either way is time consuming and subjective, thus not suitable for the large data sets generated by high-throughput flow Cytometers or the cross-lab data analysis.

Here is one xml workspace (manual gating scheme) exported from flowJo.

flowDataPath <- system.file("extdata", package = "flowWorkspaceData")
wsfile <- list.files(flowDataPath, pattern="manual.xml",full = TRUE)
wsfile
## [1] "/usr/local/lib/R/cytoset/flowWorkspaceData/extdata/manual.xml"

By usingflowWorkspacepackage, We can load it into R,

apply (flowjo_to_gatingset) themanual gatesdefined inxmlto the rawFSCfiles,

gs <- flowjo_to_gatingset(ws, name= "T-cell", subset =1, isNcdf = TRUE)

and then visualize theGating Hierarchy

and thegates:

library(ggcyto)
autoplot(gh)

This is a gating scheme for T cell panel, which tries to identify T cell sub-populations. We can achieve the same results by using automated gating pipeline provided by this package.

1.2. Automated Gating

flowCore,flowStats,flowClust and other packages provides many different gating methods to detect cell populations and draw the gates automatically.

flowWorkspace package provides the GatingSet as an efficient data structure to store, query and visualize the hierarchical gated data.

By taking advantage of these tools, openCyto package can create the automated gating pipeline by a gating template, which is essentially the same kind of hierarchical gating scheme used by the biologists and scientists.

2. Create gating templates

2.1. Template format

First of all, we need to describe the gating hierarchy in a spread sheet (a plain text format). This spread sheet must have the following columns: * alias: a name used label the cell population, the path composed by the alias and its precedent nodes (e.g. /root/A/B/alias) has to be uniquely identifiable. * pop: population patterns of +/- or +/-+/-, which tells the algorithm which side (postive or negative) of 1d gate or which quadrant of 2d gate to be kept. * parent: the parent population alias, its path has to be uniquely identifiable. * dims: characters seperated by comma specifying the dimensions(1d or 2d) used for gating. It can be either channel name or stained marker name. * gating_method: the name of the gating function (e.g. flowClust). It is invoked by a wrapper function that has the identical function name prefixed with a dot.(e.g. .flowClust) * gating_args: the named arguments passed to gating function * collapseDataForGating: When TRUE, data is collapsed (within groups if groupBy specified) before gating and the gate is replicated across collapsed samples. When set FALSE (or blank),then groupBy argument is only used by preprocessing and ignored by gating. * groupBy: If given, samples are split into groups by the unique combinations of study variable (i.e. column names of pData,e.g.“PTID:VISITNO”). when split is numeric, then samples are grouped by every N samples * preprocessing_method: the name of the preprocessing function(e.g. prior_flowClust). It is invoked by a wrapper function that has the identical function name prefixed with a dot.(e.g. .prior_flowClust) the preprocessing results are then passed to gating wrapper function through pps_res argument. * preprocessing_args: the named arguments passed to preprocessing function.

2.2. Example template

Here is the an example of the gating template.

library(openCyto)
library(data.table)
gtFile <- system.file("extdata/gating_template/tcell.csv", package = "openCyto")
dtTemplate <- fread(gtFile)
dtTemplate
##             alias    pop    parent        dims   gating_method
##  1:     nonDebris      +      root       FSC-A gate_mindensity
##  2:      singlets      + nonDebris FSC-A,FSC-H     singletGate
##  3:         lymph      +  singlets FSC-A,SSC-A       flowClust
##  4:           cd3      +     lymph         CD3 gate_mindensity
##  5:             * -/++/-       cd3     cd4,cd8 gate_mindensity
##  6: activated cd4     ++  cd4+cd8-    CD38,HLA        tailgate
##  7: activated cd8     ++  cd4-cd8+    CD38,HLA        tailgate
##  8:      CD45_neg      -  cd4+cd8-      CD45RA gate_mindensity
##  9:     CCR7_gate      +  CD45_neg        CCR7       flowClust
## 10:             * +/-+/-  cd4+cd8- CCR7,CD45RA         refGate
## 11:             * +/-+/-  cd4-cd8+ CCR7,CD45RA gate_mindensity
##               gating_args collapseDataForGating groupBy
##  1:                                          NA      NA
##  2:                                          NA      NA
##  3: K=2,target=c(1e5,5e4)                    NA      NA
##  4:                                        TRUE       4
##  5:     gate_range=c(1,3)                    NA      NA
##  6:                                          NA      NA
##  7:              tol=0.08                    NA      NA
##  8:     gate_range=c(2,3)                    NA      NA
##  9:           neg=1,pos=1                    NA      NA
## 10:    CD45_neg:CCR7_gate                    NA      NA
## 11:                                          NA      NA
##     preprocessing_method preprocessing_args
##  1:                                      NA
##  2:                                      NA
##  3:      prior_flowClust                 NA
##  4:                                      NA
##  5:                                      NA
##  6:  standardize_flowset                 NA
##  7:  standardize_flowset                 NA
##  8:                                      NA
##  9:                                      NA
## 10:                                      NA
## 11:                                      NA

Each row is usually corresponding to one cell population and the gating method that is used to get that population. We will try to explain how to create this gating template based on the manual gating scheme row by row.

2.2.1. “nonDebris”

##        alias pop parent  dims   gating_method gating_args
## 1: nonDebris   +   root FSC-A gate_mindensity            
##    collapseDataForGating groupBy preprocessing_method preprocessing_args
## 1:                    NA      NA                                      NA
  • The population name is "nonDebris" (specified in alias field).
  • The parent node is root (which is always the first node of gating hierarchy by default).
  • We use mindensity (one of the gating functions provided by openCyto package) as gating_method to gate on dimension (dim) of FSC-A.
  • As the result, it will generate a 1d gate on FSC-A. + in pop field indicates the positive side of 1d gate is kept as the population of interest.
  • There is no grouping or preprocessing involved in this gate, thus leave the other columns as blank

2.2.2. “singlets”

##       alias pop    parent        dims gating_method gating_args
## 1: singlets   + nonDebris FSC-A,FSC-H   singletGate            
##    collapseDataForGating groupBy preprocessing_method preprocessing_args
## 1:                    NA      NA                                      NA
  • The population name is "singlets" (alias field).
  • The parent node is nonDebris.
  • gating_method is singletGate (function from by flowStats package)
  • As the result, a polygonGate will be generated on FSC-A and FSC-H (specified by dims) for each sample.
  • Again, + in pop field stands for "singlets+". But here it is 2d gate, which means we want to keep the area inside of the polygon

2.2.3. “lymphocyte”

##    alias pop   parent        dims gating_method           gating_args
## 1: lymph   + singlets FSC-A,SSC-A     flowClust K=2,target=c(1e5,5e4)
##    collapseDataForGating groupBy preprocessing_method preprocessing_args
## 1:                    NA      NA      prior_flowClust                 NA
  • Similarly, alias specifies the name of population
  • parent points to singlets
  • Since we are going to use flowClust as gating_method to do the 2-dimensional gating, dims is comma separated string, x axis (FSC-A) goes first, y (SSC-A) the second. This order doesn’t affect the gating process but will determine how the gates are displayed.
  • All the parameters that flowClust algorithm accepts can be put in gating-args as if they are typed in R console. see help(flowClust) for more details of these arguments
  • flowClust algorithm accept the extra arguments priors that is calculated during preprocessing stage (before the actual gating), thus, we supply the preprocessing_method with prior_flowClust.

2.2.4. “cd3+” (Tcell)

##    alias pop parent dims   gating_method gating_args collapseDataForGating
## 1:   cd3   +  lymph  CD3 gate_mindensity                              TRUE
##    groupBy preprocessing_method preprocessing_args
## 1:       4                                      NA

It is similar to the nonDebris gate except that we specify collapseDataForGating as TRUE, which tells the pipeline to collapse all samples into one and applies mindensity to the collapsed data on CD3 dimension. Once the gate is generated, it is replicated across all samples. This is only useful when each individual sample does not have enough events to deduce the gate. Here we do this just for the purpose of proof of concept.

2.2.5. CD4 and CD8

The forth row specifies pop as cd4+/-cd8+/-, which will be expanded this into 6 rows.

##    alias    pop parent    dims   gating_method       gating_args
## 1:     * -/++/-    cd3 cd4,cd8 gate_mindensity gate_range=c(1,3)
##    collapseDataForGating groupBy preprocessing_method preprocessing_args
## 1:                    NA      NA                                      NA

First two rows are two 1d gates that will be generated by gating_method on each dimension (cd4 and cd8) independently:

##    alias pop                        parent dims   gating_method
## 1:  cd4+   + /nonDebris/singlets/lymph/cd3  cd4 gate_mindensity
## 2:  cd8+   + /nonDebris/singlets/lymph/cd3  cd8 gate_mindensity
##          gating_args collapseDataForGating groupBy preprocessing_method
## 1: gate_range=c(1,3)                                                   
## 2: gate_range=c(1,3)                                                   
##    preprocessing_args
## 1:                   
## 2:

Then another 4 rows are 4 rectangleGates that corresponds to the 4 quadrants in 2d projection (cd4 vs cd8).

##       alias pop                        parent    dims gating_method
## 1: cd4+cd8+  ++ /nonDebris/singlets/lymph/cd3 cd4,cd8       refGate
## 2: cd4-cd8+  -+ /nonDebris/singlets/lymph/cd3 cd4,cd8       refGate
## 3: cd4+cd8-  +- /nonDebris/singlets/lymph/cd3 cd4,cd8       refGate
## 4: cd4-cd8-  -- /nonDebris/singlets/lymph/cd3 cd4,cd8       refGate
##                                                              gating_args
## 1: /nonDebris/singlets/lymph/cd3/cd4+:/nonDebris/singlets/lymph/cd3/cd8+
## 2: /nonDebris/singlets/lymph/cd3/cd4+:/nonDebris/singlets/lymph/cd3/cd8+
## 3: /nonDebris/singlets/lymph/cd3/cd4+:/nonDebris/singlets/lymph/cd3/cd8+
## 4: /nonDebris/singlets/lymph/cd3/cd4+:/nonDebris/singlets/lymph/cd3/cd8+
##    collapseDataForGating groupBy preprocessing_method preprocessing_args
## 1:                                                                      
## 2:                                                                      
## 3:                                                                      
## 4:

As we see here, "refGate" in gating_method indicates that they are constructed based on the gate coordinates of the previous two 1d gates. Those 1d gates are thus considered as "reference gates" that are referred by colon separated alias string in gating_args: "cd4+:cd8+".

Alternatively, we can expand it into these 6 rows explicitly in the spread sheet. But this convenient representation is recommended unless user wants have finer control on how the gating is done. For instance, sometime we need to use different gating_methods to generate 1d gates on cd4 and cd8. Or cd8 gating needs to depend on cd4 gating ,i.e. the parent of c8+ is cd4+(or cd4-) instead of cd3. Sometimes we want to have the customized alias other than quadrant-like name (x+y+) that gets generated automatically. (e.g. 5th row of the gating template)

3. Load gating template

After the gating template is defined in the spread sheet, it can be loaded into R:

gt_tcell <- gatingTemplate(gtFile, autostart = 1L)
gt_tcell
## --- Gating Template: default
##  with  29  populations defined

Besides looking at the spread sheet, we can examine the gating scheme by visualizing it:

plot(gt_tcell)

As we can see, the gating scheme has been expanded as we described above. All the colored arrows source from the parent population and the grey arrows source from the reference population(/gate).

4. Run the gating pipeline

Once we are satisfied with the gating template, we can apply it to the actual flow data.

4.1. Load the raw data

First of all, we load the raw FCS files into R by ncdfFlow::read.ncdfFlowSet (It uses less memory than flowCore::read.flowSet) and create an empty GatingSet object.

## A GatingSet with 2 samples

4.2. Compensation

Then, compensate the data. If we have compensation controls (i.e. single stained samples), we can calculate the compensation matrix by flowCore::spillover function. Here we simply use the compensation matrix defined in flowJo workspace.

Here is one example showing the compensation outcome:

4.3. Transformation

All the stained channels need to be transformed properly before the gating. Here we use the flowCore::estimateLogicle to do the logicle transformation.

Here is one example showing the transformation outcome:

4.5. Gating

Now we can apply the gating template to the data:

gating(gt_tcell, gs)

Optionally, we can run the pipeline in parallel to speed up gating. e.g.

gating(gt_tcell, gs, mc.cores=2, parallel_type = "multicore")

4.6. Hide nodes

After gating, there are some extra populations generated automatically by the pipeline (e.g. refGate).

plot(gs[[1]])

We can hide these populations if we are not interested in them:

4.7. visualization

plot(gs[[1]])

autoplot(gs[[1]])

4.8. Apply a gating method without csv template

Sometime it will be helpful (especially to work with an already gated data) to be able to interact with he GatingSet directly without the need to write the compelete csv gating template.

5. Conclusion

The openCyto package allows user to specify their gating schemes and gate the data in a data-driven fasion. It frees the scientists from the labor-intensitive manual gating routines and increases the speed as well as the reproducibilty and objectivity of the data analysis work.